The present invention relates to a switching power supply apparatus including a step-down DC/DC converter, and more particularly, to a miniaturization technique.
As a large electricity insulative DC/DC converter with low output voltage and high output current, there is used a forward bridge converter. FIG. 1 shows a circuit diagram of a conventional switching power supply apparatus of this kind.
In FIG. 1, a series circuit having a switching Q1 including a MOSFET or the like and a switch Q2 including a MOSFET or the like is connected to both ends of a DC power supply Vdc1, and a series circuit having a switch Q3 including a MOSFET or the like and a switch Q4 including a MOSFET or the like is connected to both ends of the DC power supply Vdc1.
A series circuit having a primary winding 1a (winding number of np, exciting inductance of Lt) of a transformer T and a reactor L1 is connected between a connection of the switch Q1 and the switch Q2, and a connection of the switch Q3 and the switch Q4.
A diode Dq1 and a capacitor C1 are connected to both ends of the switch Q1 in parallel, a diode Dq2 and a capacitor C2 are connected to both ends of the switch Q2 in parallel, a diode Dq3 and a capacitor C3 are connected to both ends of the switch Q3 in parallel, and a diode Dq4 and a capacitor C4 are connected to both ends of the switch Q4 in parallel.
A primary winding 1a of the transformer T, a first secondary winding 1b (winding number of ns1), and a second secondary winding 1c (winding number of ns2) are wound in phase. The first secondary winding 1b and the second secondary winding 1c are serially connected to each other on the secondary side of the transformer T, an anode of a diode D3 is connected to one end (filled circle side) of the first secondary winding 1b, and an anode of a diode D4 is connected to one end of the second secondary winding 1c. A series circuit having a reactor Lo and a smoothing capacitor Co is connected between a connection of a cathode of the diode D3 and a cathode of the diode D4 and a connection of the first secondary winding 1b and the second secondary winding 1c. The diodes D3 and D4, the reactor Lo, and the smoothing capacitor Co constitute a rectifying smoothing circuit. The rectifying smoothing circuit rectifies and smoothens voltage (ON/OFF controlled pulse voltage) induced by the first secondary winding 1b and the second secondary winding 1c of the transformer T, and outputs the resultant voltage to a load RL.
A control circuit 100 ON/OFF controls the pair of the switch Q1 and the switch Q4 and the pair of the switch Q2 and the switch Q3 alternately at a predetermined cycle, and when an output voltage of the rectifying smoothing circuit becomes equal to or higher than a reference voltage, the control circuit 100 narrows an ON pulse width (ON period) to be applied to gates of the switches of each pair, and widens an OFF pulse width (OFF period) to be applied to gates of the switches of each pair. That is, when the output voltage of the rectifying smoothing circuit becomes equal to or higher than the reference voltage, the on-duties of the switches of each pair are narrowed, thereby controlling the output voltage to maintain a constant voltage.
An operation of the conventional switching power supply apparatus shown in FIG. 1 thus configured will be explained with reference to a timing chart shown in FIG. 2.
In FIG. 2, a reference symbol “ns1v” represents voltages of both ends of the first secondary winding 1b of the transformer T, “ns1i” represents current flowing to the first secondary winding 1b of the transformer T, “Lov” represents voltages of both ends of the reactor Lo, “Loi” represents current flowing to the reactor Lo, and “Ai” represents current flowing through a point A.
At time t0, if the switches Q2 and Q3 are turned OFF and the switches Q1 and Q4 are turned ON, current passes through a path extending along Vdc1, Q1, L1, 1a, Q4, and Vdc1. Since negative voltage is generated in one end of the first secondary winding 1b and the other end (on the side of the black circle) of the second secondary winding 1c, current ns1i does not flow through the diode D3 and the first secondary winding 1b, and the current flows through the diode D4. That is, current Loi flows through a path extending along 1c, D4, Lo, Co, and 1c. Thus, positive voltage is generated in one end (on the filled circle side) of the reactor Lo.
At time t11, if the switches Q1 and Q4 are turned OFF, voltage ns1v of the first secondary winding 1b and voltage ns2v (not shown) of the second secondary winding 1c become substantially zero, and current ns1i is also substantially zero. At that time, current Loi flows through a path extending along Lo, Co, 1c, D4, and Lo. At that time, voltage in one end of the reactor Lo becomes negative voltage.
At time t12, if the switches Q1 and Q4 are turned OFF and the switches Q2 and Q3 are turned ON, current flows through a path extending along Vdc1, Q3, 1a, L1, Q2, and Vdc1. Since positive voltage is generated in one end of the first secondary winding 1b and the other end of the second secondary winding 1c, current ns1i flows through the diode D3 and the first secondary winding 1b, and no current flows through the diode D4. That is, current Loi flows through a path extending along 1b, D3, Lo, Co, and 1b. Thus, positive voltage is generated in the reactor Lo.
At time t13, if the switches Q2 and Q3 are turned OFF, voltage ns1v of the first secondary winding 1b and voltage ns2v (not shown) of the second secondary winding 1c become substantially zero, and current ns1i is gradually reduced. At that time, current Loi flows through a path extending along Lo, Co, 1b, D3, and Lo. At that time voltage in one end of the reactor Lo is negative voltage.
An operation from time t14 and thereafter is the repetition of operations from time t0 to time t13.